An internal combustion engine is designed so that fuel explodes within the cylinder bore when the piston is positioned at top dead center, and the piston is moved downward due to the explosion. Therefore, the upper part of the cylinder bore wall increases in temperature as compared with the lower part of the cylinder bore wall. Accordingly, a difference in the amount of thermal deformation occurs between the upper part and the lower part of the cylinder bore wall (i.e., the upper part of the cylinder bore wall expands to a large extent as compared with the lower part of the cylinder bore wall).
As a result, the frictional resistance of the piston against the cylinder bore wall increases, and the fuel consumption increases. Therefore, a reduction in difference in the amount of thermal deformation between the upper part and the lower part of the cylinder bore wall has been desired.
Attempts have been made to control the cooling efficiency in the upper part and the lower part of the cylinder bore wall due to the coolant by disposing a water jacket spacer in a groove-like coolant passage to adjust the flow of the coolant in the groove-like coolant passage such that the cylinder bore wall has a uniform temperature. For example, Patent Literature 1 discloses an internal combustion engine heating medium passage partition member that is disposed in a groove-like heating medium passage formed in a cylinder block of an internal combustion engine to divide the groove-like heating medium passage into a plurality of passages, the heating medium passage partition member including a passage division member that is formed at a height above the bottom of the groove-like heating medium passage, and serves as a wall that divides the groove-like heating medium passage into a bore-side passage and a non-bore-side passage, and a flexible lip member that is formed from the passage division member in the opening direction of the groove-like heating medium passage, the edge area of the flexible lip member being formed of a flexible material to extend beyond the inner surface of one of the groove-like heating medium passages, and coming in contact with the inner surface at a middle position of the groove-like heating medium passage in the depth direction due to the flexure restoring force after insertion into the groove-like heating medium passage to separate the bore-side passage and the non-bore-side passage.